1. Field of the Invention
The invention relates generally to techniques for formation resistivity logging using induction tools. More particularly, the invention relates to induction tools and methods for reducing borehole effects in resistivity measurements.
2. Background Art
Electromagnetic (EM) induction tools are used in the oil and gas industry to determine the resistivity of earth formations surrounding a borehole. Induction tools work by using a transmitting coil (transmitter) to set up an alternating magnetic field in the earth formations. This alternating magnetic field induces eddy currents in the formations. One or more receiving coils (receivers), disposed at a distance from the transmitter, are used to detect the current flowing in the earth formation. The magnitudes of the received signals are approximately proportional to the formation conductivity. Therefore, formation conductivities may be derived from the received signals.
Conventional wireline and LWD EM induction tools are implemented with coils (antennas) that may function as sources and/or sensors. On wireline EM induction tools, the antennas are typically enclosed by a housing (or tool body) constructed of a tough plastic (insulating) material, e.g., a laminated fiberglass material impregnated with epoxy resin. On LWD EM induction tools, the antennas are generally mounted on metallic supports (collars) to withstand the hash environments encountered during drilling.
On both wireline and LWD induction tools, the antennas are typically spaced apart from each other along the axis of the tool. These antennas are generally coils of the solenoid type that comprise one or more turns of insulated conductor wire wound around a support. U.S. Pat. Nos. 4,873,488 and 5,235,285 (both assigned to the present assignee), for example, disclose instruments equipped with antennas disposed along a central metallic support (a conductive mandrel).
A coil (or antenna) carrying a current can be represented as a magnetic moment proportional to the current and the area. The direction and magnitude of the magnetic moment can be represented by a vector perpendicular to the plane of the coil. In conventional induction and propagation logging instruments, the transmitter and receiver antennas are mounted with their magnetic moments aligned with the longitudinal axis of the instruments. That is, these instruments have longitudinal magnetic dipoles (LMD). When an LMD tool is placed in a borehole and energized to transmit EM energy, the induced eddy currents flow in loops around the antenna in the borehole and in the surrounding formation. These eddy currents flow in planes that are perpendicular to the tool axis (hence, borehole axis). Therefore, no eddy current flows up or down the borehole when the tool is centralized in the borehole.
An emerging technique in the field of EM induction well logging is the use of instruments incorporating antennas having tilted or transverse antennas, i.e., the magnetic dipoles of the antennas are tilted or perpendicular to the tool axis. That is, these instruments have transverse or tilted magnetic dipoles (TMD). These TMD instruments can induce eddy currents that flow up and down the borehole and, thus, provide measurements that are sensitive to dipping planes, formation fractures, or formation anisotropy. Modern induction tools typically include triaxial arrays, in which the transmitter and receivers may each comprise three coils arranged in different orientations (typically in orthogonal directions). Two of the coils in a triaxial transmitter or receiver may be TMD antennas. Logging instruments equipped with TMDs are described, for example, in U.S. Pat. Nos. 4,319,191, 5,508,616, 5,757,191, 5,781,436, 6,044,325, and 6,147,496.
While the TMD tools (including triaxial tools) are capable of providing additional information about the formation resistivity, these tools are more strongly affected by the borehole, particularly in high contrast situations, i.e., when the mud in the borehole is more conductive than the formation. When a TMD tool is energized at the center of a borehole (shown as 20 in FIG. 1a), it can induce eddy currents flowing up and down the borehole. However, due to the symmetry, the up and down currents cancel out and there is no net current flowing up or down the borehole. When a TMD tool is eccentered, the symmetry may disappear. If the TMD tool is eccentered in a direction parallel to the direction of the magnetic dipole of its antenna (shown as 22 in FIG. 1a), the symmetry with respect to the antenna is maintained and there is still no net current flowing along the borehole axis, when the antenna is energized. However, if a TMD is eccentered in a direction perpendicular to the direction of the magnetic dipole of its antenna (shown as 21 in FIG. 1a), the symmetry no longer exists and there will be net currents flowing up or down the borehole, when the antenna is energized. In high contrast situations (i.e., conductive mud and resistive formation), the borehole currents can flow a long distance along the borehole. The current flow in the formation will also be asymmetric in this case. These asymmetric currents induce undesired signals in the TMD receivers that can be many times larger than the expected signals from the formation.
U.S. Pat. No. 5,041,975 (assigned to the present assignee) discloses a technique for processing data from downhole measurements to correct for borehole effects. U.S. Pat. No. 5,058,077 discloses a technique for processing downhole sensor data to compensate for the effect of eccentric rotation on the sensor while drilling. U.S. Pat. No. 6,541,979 (assigned to the present assignee) discloses techniques for reducing the effect of borehole eccentricity, using mathematical corrections for the borehole currents effects.
U.S. Pat. No. 6,573,722 (assigned to the present assignee) discloses methods to reduce the effect of tool eccentricity in the borehole by minimizing the borehole currents passing the TMD antennas. This patent is hereby incorporated by reference. In one embodiment, an electrode located below the TMD antenna is hard-wired to another electrode located above the TMD antenna to provide a conductive path behind the TMD antenna. This additional conductive path reduces the amount of borehole currents passing in front of the TMD antenna, and thus minimizes the undesirable effects. In another embodiment, a tool is disclosed that generates a localized current in the borehole (between the two electrodes located on either side of a TMD antenna) that counteracts or cancels out the undesirable borehole currents. Further examples of methods and apparatus for reducing the borehole current effects include U.S. Pat. Nos. 6,573,722 B2, 6,624,634 B2, 6,693,430 B2, 6,680,613 B2, 6,710,601 B2, all of which are issued to Rosthal et al. and assigned to the assignee of the present invention, and published U.S. Patent Applications Ser. Nos. 2003/0146753 A1 and 2003/0155924 A1, both of which are by Rosthal et al. and assigned to the assignee of the present invention.
While these prior art methods are effective in reducing borehole effects on induction tools, there remains a need for further improvements in the design of induction tools that are less affected by tool eccentricity in the borehole. Experimental studies showed that the strategy of canceling current flow up and down the borehole did not give satisfactory performance. Large electrodes can produce a temperature-dependent error signal, so it is preferable to use small electrodes.